Except for the "variable.less" @import statement, all other @imports are pointing to a inexistent bootstrap folder inside the sub-theme.
So my nobrain workaround was download the files from http://getbootstrap.com/ and create this folder with the LESS folder inside it to fullfill the sub-theme's expectations.

Method 1: Bootstrap Source Files

Download and extract the Bootstrap 2.3.2 sourceinto your new sub-theme. After it has been extracted, the folder should read bootstrap. If for whatever reason you have an additional bootstrap folder wrapping the the bootstrap folder (like: bootstrap/bootstrap), remove the wrapping bootstrap folder. You will not need to touch these files again. This allows the framework to be updated in the future.

In the sub-theme's variables.less file, the beginning of all those imports is: ../, which is the parent directory. In this case, it would be the base sub-theme folder, not the less folder.

There is also a difference in downloading the Bootstrap framework's source and it's compiled release. Using the LESS module or even a local LESS preprocessor will require the source files: the ones ending in .less.